Shot averaging for fine pattern alignment with minimal throughput loss

ABSTRACT

A way to average alignment measurements that obtains the advantage of multiple alignment marks per shot without requiring actual measurement of all alignment marks on all wafers of a batch. All alignment marks on all sampled shots are measured and averaged on the first wafer of a batch. The offset between a single sampled alignment mark and the average total offset for the wafer is characterized and applied when that alignment mark is sampled on succeeding wafers.

This application is a division of Ser. No. 09/909,226, filed Jul. 19,2001, which claims the benefit of Provisional application Ser. No.60/222,208, filed Aug. 1, 2000.

BACKGROUND AND SUMMARY OF THE INVENTION

The present application relates to integrated circuit fabrication, andmore specifically to photolithographic processes used in IC fabrication.

BACKGROUND

Photolithography is a major element of IC fabrication. Severaltechnologies have been developed to produce resist patterns small enoughto meet the size requirements of the industry. Ever decreasing sizerequirements mean that smaller and smaller resist patterns must beplaced on wafer surfaces to etch the necessary features. Alignment ofwafers has therefore become an important part of IC fabrication, sincemisalignments that once were within pattern tolerance are no longeracceptable.

Alignment of wafers is accomplished in a variety of ways. Typically,marks are placed on wafer surfaces that indicate the correct placementof the wafer within the exposure system, so that when the wafer isexposed, the exposure pattern will correctly lie with respect toprevious underlying patterns already present on a wafer. In someapplications, where gates and other features require extremely precisepattern alignment, errors on the order of tens of nanometers must becorrected. This requires an alignment procedure capable of consistentlyplacing patterns to within such a tolerance.

High resolution lithographic tools, such as steppers and step-and-scantools, are used to print multiple patterns, or shots, on a substrate.Though methods vary, generally a stepper samples several shots, orsingle exposures of a pattern on the wafer surface, to check the waferalignment. According to the overall map for the chip pattern, severalshots are sampled, each containing alignment information in the form ofone or more alignment marks. Steppers use software “recipes,” where aseparate family of recipes exists for each device being fabricated, andwhere each recipe samples certain shots to optimize alignment for thatparticular device.

Alignment marks determine the offset needed to properly align the wafer,typically in cartesian coordinates. There will often be separate marksfor x-direction and y-direction orientation, though combinational marksalso exist that align the wafer in both directions with a single markstructure. The information gathered from the alignment marks can be inmany forms, such as a digital image of the mark or an interferencepattern corresponding to the mark's orientation and location. The exactmethod depends on many factors, such as the material used,planarization, etc. Different device fabrication procedures anddifferent exposure systems therefore use different sampling modes togather information from the alignment marks.

Alignment systems in general are designed for a given device fabricationusing a given pattern exposure system. Though the optics used in ICfabrication exposure are among the best in the world, distortion stillexists, and as device sizes decrease even the best optics availableproduce distortion that can cause pattern alignment failure.

During printing of a device, a given level is printed and alignmentmarks are made for orienting and aligning the wafer when a later levelof the device is exposed. Each subsequent pattern is aligned to marks onthe wafer which were printed at one of the previous pattern steps.However, during printing, the patterns may be minutely displaced in thex- and y-direction, depending on their positions in the imaging fieldand on the lens distortion associated with that position. Thisdisplacement is controlled as much as possible, but remains significantcompared to the alignment tolerance. Alignment marks themselves alsoexperience this displacement, depending on which area of the lens isused to print each mark on a given device. This arises from the factthat different sections of the lens produce different distortion in animage.

One solution to this problem has been to compile databases ofinformation on every device type (numbering in the hundreds) and everyexposure system (often fifteen or more) to create custom offsets thatpredict and compensate for the distortions of a given device layer.These databases quickly grow to immense size, and often are useless whenchanges are made to device processes or new devices are introduced.

In recent alignment systems, several alignment marks are sampled fromeach of several shots, and then averaged to produce a total alignmentfor the wafer. Though this method minimizes the need for custom offsetdatabases, it is time consuming. A wafer spends a certain amount of timeon the tool for preparation (including alignment of the wafer),printing, and overhead tasks. As an example, alignment for only one xand y alignment mark per shot on 8 shots per wafer may take 20 secondsout of a total 90 seconds on the wafer in a typical system. If eightmarks are used for averaging, the alignment time increases to 160seconds, adding 140 seconds to the entire process and reducingthroughput to 40%.

The IC fabrication art would therefore benefit from a process whichobtained the advantages of multi-sample averaging without the drasticreduction in throughput.

Shot Averaging for Fine Pattern Alignment with Minimal Through-Put Loss

The present application discloses a method of shot averaging that usesinformation gathered from the first wafer of a batch to derive alignmentmark relationships on later wafers. This innovative process alleviatesthe need to measure the locations of all alignment marks on succeedingwafers of a batch.

Advantages of the disclosed methods and structures, in variousembodiments, can include one or more of the following:

fewer measurements taken of alignment marks;

increase in throughput without loss of alignment accuracy;

the advantages of alignment mark averaging across a shot is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIGS. 1A, 1B, and 1C show a wafer and patterns with alignment marks.

FIG. 2 shows a flowchart for the preferred embodiment.

FIG. 3 shows the reduction in measurements required for alignment of awafer batch.

FIG. 4 shows a wafer fabrication process using the innovations of thepresent application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily delimit anyof the various claimed inventions. Moreover, some statements may applyto some inventive features but not to others.

The process is an improvement to alignment shot averaging, a practicewhich is known in the relevant art. Alignment of wafers forphotolithography is typically done by sampling alignment marks that arepresent on several different exposures, or “shots,” of a wafer surface.Each shot contains multiple alignment marks, and by measuring offseterrors for multiple marks per sampled shot, an average offset can beobtained that is more representative of the average offset than samplingonly a single mark per sampled shot.

In the preferred embodiment of a process implementing the innovations ofthe present application, all alignment marks are measured on each shotthat is sampled for alignment offset in only the first wafer of a batch.Each corresponding alignment mark from each sampled shot is averaged,producing several average offsets. For example, if four marks are usedon each shot, there will be four average offsets, one for each mark.These average offsets are then averaged to obtain a total offset forthat wafer.

Next, the difference between one of these average offsets (i.e., theaverage offset obtained by averaging ONE mark from each shot) ischaracterized in terms of the total offset. This produces a value thatcontains the relationship between the averages of the four marks on eachsampled shot. Using this value, the relationship between the alignmentmarks on succeeding wafers is incorporated into the alignment of thesucceeding wafers without having to actually measure those marks. Onlyone mark from each sampled shot need be measured on succeeding wafers,and using the carried forward value from the first wafer, the totaloffset is determined.

This innovative method of shot averaging works because of the systematicnature of the errors introduced to alignment during photolithography.For a given batch of wafers, the relationship between the averagealignment mark offsets is constant. That is, the average offset foralignment mark #1 (found by averaging the offsets of all #1 alignmentmarks from each shot) with respect to the average offsets for alignmentmarks #2, #3, and #4 is constant. Therefore, this relationship isdetermined from measurements taken on the first wafer only, therelationship is characterized and carried forward to help alignsucceeding wafers, where this relationship holds. Therefore, all markson each sampled shot need only be sampled on the first wafer to obtainthe relationship between the marks. Succeeding wafers in a batch willrequire sampling of only one alignment mark per sampled shot, and usingthe relationship learned from the first wafer, the offset for thesucceeding wafers can be derived as shown below.

FIG. 1 shows a wafer 102. The wafer 102 contains many exposures 104 ofthe pattern. These exposures, or shots, contain alignment marks 106, asshown in the detail. These marks 106 are labeled x1, x2, x3, and x4.Each shot contains multiple alignment marks which serve to orient thewafer for exposure. Multiple shots are sampled, marked with an “x” onFIG. 1. In ordinary systems, measurement data from all marks on allshots are averaged to compensate alignment. This averaging of multiplemarks within each shot reduces the error associated with each markbecause of its unique position in the shot.

This figure only shows one example of a mark. Mark designs vary widely,with some marks giving both x and y alignment information. Any markdesign will work with the present innovations.

All x1s are averaged to get an average value x1bar. Likewise, averagesare obtained for the other three marks on each shot, yielding x2bar,x3bar, and x4bar. The total correction needed for the wafer in the xdirection is therefore the average of these averages as described in thefollowing equation:

Equation 1

{double overscore (x)}=({overscore (x₁)}+ {overscore (x₂)}+ {overscore(x₃)}+ {overscore (x₄)})/4

Thus xbarbar for wafer 1 indicates the total offset in the x-directionfor wafer 1 alignment. A similar calculation for the y coordinates isalso performed, yielding a total offset in the y-direction.

The relationship between x1bar, x2bar, x3bar, and x4bar is constantbetween wafers in the batch, so their values can be incorporated intothe total correction for the second wafer by using data from the firstwafer as follows.

Equation 2

{overscore (x)} ₁ _(wafer1) −{double overscore (x)}_(wafer1)=carryforward

For succeeding wafers in the batch, only one alignment mark per sampledshot is measured and included in the average, yielding a value for x1baronly. Using the value in Equation 2 in the following expression yieldsthe total offset for wafer 2 in the x-direction.

Equation 3

{double overscore (x)} _(wafer2) ={overscore (x)} ₁ _(wafer2)−({overscore (x)} ₁ _(wafer1) −{double overscore (x)} _(wafer1))

The value x1barwafer1−xbarbarwafer1 contains the relationship betweenthe alignment marks on the shots. This relationship does not change whenwafers are switched, so it can be used with all wafers of a batch toreduce the number of samples that must be taken per wafer. Using thiscarryforward value, only one alignment mark sample need be taken pershot on each wafer, greatly reducing alignment time for each wafer.

The second wafer in the batch is processed next. Only one x-directionwafer alignment mark from each shot need be measured and averaged,yielding x1bar for wafer 2.

The process of the preferred embodiment is depicted in FIG. 2. Thisexample assumes a wafer batch of 24 wafers, wherein 12 shots are takenof each wafer, with 4 alignment marks per shot. As a simplification,alignment of the wafers will be explained in only one dimension. Ofcourse, either the same process would need to be repeated to align thewafer in a second, preferably orthogonal direction, or else thealignment marks must incorporate enough data to align the wafer in bothdirections at once.

In Step 1, measure the four alignment marks for each of shots 1-12 onwafer 1. In Step 2, average the offsets for each alignment mark,yielding x1bar, x2bar, x3bar, x4bar for wafer 1 (assuming 4 marks pershot—where there are n marks per shot, there would be n averageoffsets). In Step 3 calculate average total offset for wafer 1 usingEquation 1. In Step 4, complete processing of wafer 1, includingexposure and overhead tasks. In Step 5, calculate the carry forward fromthe wafer 1 data, using Equation 2, to be used in aligning allsubsequent wafers. In Step 6, measure only the first alignment mark foreach of shots 1-12 on wafer 2 (note that other embodiments might choosea subset of marks for subsequent wafers rather than just one mark). InStep 7, average the alignment mark offsets yielding x1bar for wafer 2.In Step 8, calculate the average total offset for wafer 2 using Equation3. In Step 9, complete processing of wafer 2. In Step 10, repeat Steps6-9 for the remaining 22 wafers.

FIG. 3 shows the reduction in alignment samples required using thepreferred embodiment as opposed to previous alignment methods. Thisfigure shows only one possible implementation of the presentinnovations. In previous methods, several alignment marks (4 in thisexample) are measured for each of several shots (12 in this example)taken of each wafer of a batch (24 in this example). This results in1152 alignment measurements taken for the batch. In the preferredembodiment, all 4 alignment marks are measured for each of the 12 shotsonly on the first wafer, and this data is used to reduce the number ofmarks measured on succeeding wafers. As shown, only one mark is measuredon each of the 12 shots for the remaining wafers 2-24, resulting in atotal of only 324 alignment measurements for the batch.

FIG. 4 shows a block diagram of a wafer fabrication process implementingthe innovations of the present application. In Step 1 the wafer isaligned. In Step 2 a layer is printed on the wafer. This step includesexposure of the pattern, developing, etch, and possibly other stepsdepending on the specific device fabricated. In Step 3, overhead tasksare performed on the wafer. The process is repeated for succeedingwafers of a batch.

An illustrative example, using fabricated numbers, follows in order toexplain the preferred embodiment. For simplicity, only the x-directionoffset is shown.

On the first wafer of a batch, four alignment marks, x1, x2, x3, x4, aremeasured on each sampled shot. The multiple x1 values from each sampledshot are averaged to obtain a value x1bar. Similarly, x2bar, x3bar, andx4bar represent the averages of the other alignment marks. Assume thatthe following values are found.

{overscore (x₁)}=+10

{overscore (x₂)}=−5

{overscore (x₃)}=−10

{overscore (x₄)}=+5

The total average offset for the first wafer, xbarbar, is the average ofthese values, which is zero. This represents the total x displacementrequired to align the wafer in nanometers. (Note that y-directionoffsets would be obtained in a similar way.)

Next, the relationship between one of these values (x1bar, for example)is characterized in relation to the total offset for wafer 1 as follows.

{overscore (x)} ₁ _(wafer1) −{double overscore (x)}_(wafer1)=carryforward

10−0=10

This carryforward includes the relationship between x1bar and the otheraverages (i.e., x2bar, x3bar, x4bar). This relationship remains constantbetween wafers in a batch. So by using this carryforwared value and themeasurement of one alignment mark on succeeding wafers, the total offsetcan be obtained for the succeeding wafers instead of measuring all thealignment marks on succeeding wafers.

Moving to the second wafer, the average offset for only one alignmentmark is measured.

{overscore (x₁)}=20

Using the carryforward, the total offset for wafer 2 is obtained asfollows.

{double overscore (x)} _(wafer2) ={overscore (x)} ₁ _(wafer2)−carryforward

{double overscore (x)} _(wafer2)=20−10=10

As a check, we can also calculate xbarbarwafer2 using the averageoffsets from each alignment mark. If we had measured these averageoffsets, we would expect the values below:

{overscore (x₁)}=20

{overscore (x₂)}=5

{overscore (x₃)}=0

{overscore (x₄)}=15

{double overscore (x)} _(wafer2)=(20+5+0+15)/4=10

This produces the same total offset for wafer 2 as was calculated usingthe carryforward from the wafer 1, measurements.

The present innovative shot averaging method allows shot averaging withminimal throughput loss. Minimal samples are taken on all but the firstwafer, and the information gathered from the first wafer is used toreduce the number of samples taken on succeeding wafers. This greatlyreduces the time spent aligning wafers and increases throughput withoutincreasing complexity of the process. The process is implemented in thepreferred embodiment as logic in the wafer stepper software. It cancontain many variable features, such as how many wafers to use todetermine the carryforward value (1 in the examples of the presentapplication) or how many marks to measure per shot on preliminary aswell as subsequent wafers.

Definitions:

Following are short definitions of the usual meanings of some of thetechnical terms which are used in the present application. (However,those of ordinary skill will recognize whether the context requires adifferent meaning.) Additional definitions can be found in the standardtechnical dictionaries and journals.

Alignment: correct positioning of a wafer so that successive levels willlie properly with respect to one another within design tolerances.

Batch: a term referring to a number of wafers. Wafers would be of thesame batch if they share enough similarity in process so that theinnovative alignment method would work. Batches may include lots ofwafers, or multiple lots forming one or more trains of lots.

Carryforward: a calculated value containing the relationships betweenone or more average offsets and the total offset for a wafer.

Exposure: a part of the printing process where one or more patterns areplaced, through optic means, on a wafer.

Offset: the difference between the actual location of an alignment mark(or a wafer) and the desired location.

Shot: a single copy or iteration of a pattern on a wafer. Each wafertypically contains many shots.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given, but is only defined by the issued claims.

In one class of alternative embodiments, the averaging of the marks isaccomplished in a different order, or uses different value. Forinstance, rather than averaging corresponding alignment marks from eachsampled shot, all alignment marks from a given shot might be averaged,followed by averaging of the multiple average shot offsets.

In another alternative embodiment, an assumption could be made that theoffset relationship holds between shots (as well as between samples on ashot) and the process would only measure one shot per wafer after thefirst wafer, using information from the first wafer to derive the offsetrelationships between shots. Though such a process might sacrificequality for throughput, it is within the contemplation of the presentapplication.

In another class of alternative embodiments, more than one mark (butfewer than all marks) can be measured on subsequent wafers fordetermining the average offset. Likewise, more than one mark's averageoffset could be used in determining the carryforward value.

Other modifications include selectively choosing which mark to use inaveraging, considering elements such excessive distortion in certainareas of the shots or wafers or other irregularities that might throwaverages off. Certain alignment marks might also be excluded for thesame reasons.

The presently preferred embodiment described the present innovations interms of a stepper system. In the case of a step-and-scan system,similar assumptions may be made, about a mark's offset in relation tothe field of exposure for a step-and-scan system. In both cases the ideais to find offsets that are repeatable. In a stepper system, the errorsare caused mainly by lens imperfections, whereas in a step-and-scansystem the errors are caused by the convolution of the lens and scanningstages.

Combinational marks, giving both x-direction and y-direction orientationare also compatible with the present innovations. Also, instead of asingle mark measured on subsequent wafers' shots, a subset of markscould be used, which would still decrease the total number ofmeasurements taken.

In another variation, the carryforward value (or an equivalent) could bestored somewhere as a reference for future runs, alleviating the need torepeat average offset measurements of all alignment marks on the firstwafer of a run.

Additional general background, which help to show the knowledge of thoseskilled in the art regarding variations and implementations of thedisclosed inventions, may be found in the following documents, all ofwhich are hereby incorporated by reference: HANDBOOK OFMICROLITHOGRAPHY, MICROMACHINING, AND MICROFABRICATION ed. P.Rai-Choudhury (1997).

What is claimed is:
 1. A wafer alignment system, comprising: means fortaking shots of a first plurality of alignment marks on a wafer; meansfor sampling said first plurality of alignment marks on a plurality ofsampled shots on a first wafer to thereby align said first wafer; andmeans responsive to said sampling of said first plurality of alignmentmarks on said first wafer to permit sampling of fewer than said firstplurality of marks on a plurality of sampled shots on succeeding wafersto thereby align said succeeding wafers.
 2. The system of claim 1wherein said first plurality of marks is four marks.
 3. A system forwafer alignment, comprising: means for taking shots or a first pluralityof alignment marks on a wafer; means for measuring said plurality ofalignment marks on each of a plurality of sampled shots on a first waferto thereby align said first wafer; and means responsive to said meansfor measuring for measuring only one mark on each of a plurality ofsampled shots on a second wafer to thereby align said second wafer. 4.The system of claim 3, wherein said means responsive aligns a secondwafer using information gathered from alignment of said first wafer. 5.A system for shot averaging, comprising: means for taking shots of a setof alignment marks on a wafer; means for characterizing the offsetbetween said set of alignment marks with respect to the total offset ofsaid wafer to produce a value; and means responsive to thecharacterization of said offset of said wafer to align further wafersusing said value to align succeeding wafers.
 6. A system for waferalignment, comprising: means for taking shots of a set of alignmentmarks on a first wafer and a second wafer; means for sampling multiplealignment marks on each of all sampled shots of a first wafer to producemultiple offset values; means for averaging said multiple offset valuesof said marks to produce a total offset for said first wafer andcharacterizing a relationship between a single mark on each of allsampled shots on said first wafer and said total offset for said firstwafer to produce a first value and responsive to said first valuesampling a single alignment mark on each of all sampled shots on asecond wafer to produce a second value and using said first value andsaid second value to calculate a total offset for said second wafer. 7.The system of claim 6 wherein said first value is the difference betweenthe average of said single marks on each of all sampled shots on saidfirst wafer and said total offset for said first wafer.
 8. A system forwafer alignment, comprising: means for taking shots of a set ofalignment marks on a first wafer and a second wafer; means for measuringm offsets on each of n marks on each of m sampled shots on a firstwafer, for each of said n marks on each of m sampled shots, calculatingan average offset, producing n average offsets, calculating a totaloffset for said first wafer by averaging said n average offsets andcharacterizing the difference between one of said n average offsets andsaid total offset for said first wafer to produce a first value and, onsaid second wafer, measuring a plurality of offsets of one mark on eachsampled shot and calculating an average offset from said plurality ofoffsets and, using said first value and said average offset of saidsecond wafer, calculating a total offset for said second wafer.
 9. Thesystem of claim 8, wherein said n marks number
 4. 10. A fabricationsystem, comprising: means for aligning a wafer by characterization ofthe offset between a set of alignment marks with respect to the totaloffset of said wafer to produce a value, and using said value to alignsubsequent wafers; and printing a pattern on a wafer of said batch. 11.The system of claim 10 wherein said batch includes multiple lots ofwafers.
 12. A wafer alignment system, comprising: an exposure systempositioned to expose a pattern on a wafer; wherein said wafer is alignedwith respect to said exposure system by characterizing the offsetbetween a subset of alignment marks located on said wafer with respectto the total offset of said wafer to produce a value, and using saidvalue to align subsequent wafers.